Snap refresh error x509: certificate signed by unknown authority


we are using a ubuntu 18.04 server with snap installed.
If we try to refreh our snaps (core and rocketchat) we get the error
x509: certificate signed by unknown authority

I did a apt get update and apt-get upgrade and all packages are up to date.
I searched for this error and i found some things about self sigend certificates.

but if i do a snap refresh the only ssl certificate that is checked, is the ssl certificate and the trusted root certificates on the ubuntu server should be refreshed automatically, isnt it?

How can i solve the problem because i need to update our rocketchat version.

Thank you very much.
Best regards

Problem solved

The file /etc/ca-certificates.conf was modified and all mozilla root CAs are removed with a “!” at the beginning of the line.
I removed all “!” from the lines and ran update-ca-certificates. now all root certificates are copied to /etc/ssl/certs. After that i restart the snapd and the snaps can be refrehsed.